[TowerTalk] CMC-230-5K

[Jim Brown]

On Thu,4/21/2016 11:12 AM, Tod wrote:
> As a matter of curiosity:
>
> You have a (CMC) Common Mode Choke in a feed line.  You add another, identical, choke in line.
>
> If the first choke attenuates the common mode current by 20 dB, the what is the total attenuation after you add the second Choke?

That depends on how the two chokes affect common mode voltage and 
current as a result of their placement, and there is no short answer. 
First, the attenuation of a single choke depends entirely on the common 
mode circuit into which it is inserted, and that circuit varies widely 
with frequency for any given system. The variables include the imbalance 
of the antenna, the electrical length of the feedline, the termination 
of the feedline at the transmitter, the routing of the feedline from the 
antenna, etc. A really good choke could provide as little as a few dB or 
as much as 20 dB, depending on those variables. 20dB is a LOT of 
attenuation, and a very good choke would provide that only if the common 
mode voltage was quite high. In that situation, a second identical choke 
in series would increase attenuation by no more than 6 dB.

73, Jim K9YC